[IPRA][ARM] Make use of the "returned" parameter attribute

ARM has code to recognise uses of the "returned" function parameter
attribute which guarantee that the value passed to the function in r0
will be returned in r0 unmodified. IPRA replaces the regmask on call
instructions, so needs to be told about this to avoid reverting the
optimisation.

Differential revision: https://reviews.llvm.org/D64986

llvm-svn: 366669